External view of the JUNO liquid scintillator detector inside the water pool before filling.

The Jiangmen Underground Neutrino Observatory (JUNO) international collaboration team recently published a research paper titled Measurement of reactor neutrino oscillation with the first JUNO data as a cover story in the prestigious international journal Nature.

The work presents the most precise measurements of the neutrino oscillation parameters θ₁₂ and Δm²₂₁ to date, improving precision by a factor of 1.6 compared to the combined results of multiple experiments over the past few decades.

As a member institution of the JUNO international collaboration team, Xi'an Jiaotong University (XJTU) provided critical technical support for this achievement. Faculty members Zhang Qingmin, Guo Yuhang, Zhu Kangfu, Liu Yankai, Shao Zhuang, and Lyu Zhipeng from the School of Energy and Power Engineering are co-authors of the paper.

The guide tube calibration system (GTCS), developed by Professor Zhang Qingmin's team, is currently the world's largest detector calibration system and serves JUNO's high-precision energy reconstruction.

As an invasive measurement device, a calibration system must minimize its effect on the detector itself. Therefore, the GTCS was engineered to meet three strict criteria: low optical interference, low structural shadowing, and low background contribution. To achieve this, the tube diameter had to be as small as possible, the installation structure had to be extremely compact, and the materials were selected to ensure both low light absorption and low radioactivity.

From the start of development in 2015 to its final installation and delivery in 2025, the XJTU team dedicated 10 years to systematically overcoming five core challenges:

1. The calibration algorithm challenge

JUNO requires the uncertainty of its absolute energy scale to be better than 1 percent. For a liquid scintillator detector exceeding 35 meters in diameter, this requires an incredibly precise energy response model, with boundary region correction being a critical hurdle. However, gamma rays generate full-energy peaks that overlap with the Compton plateau at the detector's edge, making it difficult for traditional methods to extract accurate peak positions.

The team developed a method to precisely extract full-energy peak positions under severe overlap conditions and established an accurate reconstruction method for the meridional response curve based on limited calibration points. By further incorporating multi-parameter fitting constraints, a boundary energy spectrum response model, and interpolation reconstruction, they achieved high-precision reconstruction.

Before the Daya Bay Reactor Neutrino Experiment was decommissioned at the end of 2020, the team validated its method in a real detector environment. The algorithm met JUNO's stringent requirements and has been applied to the high-precision calibration of the detector's boundary.

2. The positioning accuracy challenge

To ensure the precise positioning of the calibration source over long distances, the team used servo motors to finely control the wire spooling step-by-step and incorporated position sensors for closed-loop control. On-site testing at JUNO demonstrated that the positioning error of the calibration source was kept within 6 centimeters across a 100-meter travel distance.

3. The stable operation challenge

The team addressed stability challenges related to bending, suspension, and tension control in the long guide tubes. Neutrino signals are incredibly weak, meaning excessive background noise can easily drown out real events.. They utilized Teflon to minimize friction, designed tension monitors for real-time tracking, and invented a tension retainer to ensure optimal tension throughout the deployment and retraction process.

The system uses a dual-end drive; even if the line snaps on one end, it can be pulled out from the other, and various thread-recovery backup plans were pre-engineered. Additionally, the team built the "JUNO guide tube calibration system monitoring platform" equipped with anomaly warnings and control capabilities.

4. The background control challenge

Neutrino signals are extremely weak, meaning excessive background noise can obscure genuine events. JUNO stipulates that the total background must be below 10 Hz, with a quota of just 0.1 Hz allocated to the GTCS.

The team screened all materials from the source. The 126-meter-long guide tube was fabricated from transparent Teflon to minimize background contribution and mitigate impacts on light reflectivity. With an outer diameter of just 19 mm, it was negligible relative to the scale of the detector, achieving the low-structural-shadowing design goal.

The system used Teflon wherever possible, and all stainless-steel components were verified low-background steel, all aimed at minimizing the system's footprint on the overall detector. Through Monte Carlo simulations and ultra-low radioactivity testing, the team kept the system's actual background contribution below 0.085 Hz, beating the 0.1 Hz requirement.

Furthermore, the GTCS water circulation system injects ultrapure water to eliminate low-light interference caused by friction during source carrier transport, providing a pristine environment for JUNO's faint-light measurements.

5. The on-site installation challenge

The guide tubes had to be installed synchronously with the detector, a lengthy and complex process where any accident could crush the tubing. To mitigate this, the team designed a dedicated repair device.

The laboratory is located 700 meters underground and kilometers away from the nearest town, making every trip down underground physically demanding. Team members routinely survived on boxed meals, and once in their cleanroom suits, they minimized water intake to avoid restroom breaks.

Because installation had to align with the detector's assembly timeline, the team sometimes waited over 10 days for an installation window; once a window opened, the work had to proceed continuously under tight time constraints.

The on-site conditions were harsh: the detector's surface was slippery, requiring high-altitude work while strapped into safety ropes. Each installation involved climbing dozens of meters, and the platforms were so cramped that workers could not stand up straight. Every single step was rehearsed repeatedly. Finally, the guide tubes were safely laid out and the system was fully commissioned.